A typical iron atom has 56 times the mass of a typical hydrogen atom. Iron is the most abundant metal, and is believed to be the tenth most abundant element, in the universe. Iron is also the most abundant (by mass, 34.6%) element making up the Earth; the concentration of iron in the various layers of the Earth ranges from high at the inner core to about 5% in the outer crust; it is possible the Earth's inner core consists of a single iron crystal although it is more likely to be a mixture of iron and nickel; the large amount of iron in the Earth is thought to contribute to its magnetic field. Its symbol Fe is an abbreviation of ferrum, the Latin word for iron.

Iron is a metal extracted from iron ore, and is hardly ever found in the free (elemental) state. In order to obtain elemental iron, the impurities must be removed by chemical reduction. Iron is used in the production of steel, which is not an element but an alloy, a solution of different metals (and some non-metals, particularly carbon).

The nucleus of iron has the highest binding energy per nucleon, so it is the heaviest element that is produced exothermically through fusion and the lightest through fission. When a star that has sufficient mass to produce iron does so, it can no longer produce energy in its core and a supernova will ensue.

Cosmological models with an open universe predict that there will be a phase where as a result of slow fusion and fission reactions, everything will become iron.

Cast iron contains 2% – 3.5% carbon and small amounts of manganese. Contaminants present in pig iron that negatively affect the material properties, such as sulfur and phosphorus, have been reduced to an acceptable level. It has a melting point in the range of 1420-1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together. It is extremely strong, hard and brittle. Working cast iron, even white hot cast iron, tends to break the object.

Wrought iron contains less than 0.5% carbon. It is a tough, malleable product, not as fusible as pig iron. It has a very small amount of carbon, a few tenths of a percent. If honed to an edge, it loses it quickly.

By 3000 BC to 2000 BC, increasing numbers of smelted iron objects (distinguishable from meteoric iron by the lack of nickel in the product) appear in Mesopotamia, Anatolia, and Egypt. However, their use appears to be ceremonial, and iron was an expensive metal, more expensive than gold. In The Illiad, weaponry is mosty bronze, but iron ingots are used for trade. Some resources (see the reference What Caused the Iron Age? below) suggest that iron was being created then as a by-product of copper refining, as sponge iron, and was not reproducible by the metallurgy of the time. By 1600 BC to 1200 BC, iron was used increasingly in the Middle East, but did not supplant the dominant use of bronze.

Axe of iron from Swedish Iron Age, found at Gotland, Sweden

In the period from the 12th to 10th century, there was a rapid transition in the Middle East from bronze to iron tools and weapons. The critical factor in this transition does not appear to be the sudden onset of a superior ironworking technology, but instead the disruption of the supply of tin. This period of transition, which occurred at different times in different parts of the world, is the ushering in of an age of civilization called the Iron Age.

Concurrent with the transition from bronze to iron was the discovery of carburization, which was the process of adding carbon to the irons of the time. Iron was recovered as sponge iron, a mix of iron and slag with some carbon and/or carbide, which was then repeatedly hammered and folded over to free the mass of slag and oxidise out carbon content, so creating the product wrought iron. Wrought iron was very low in carbon content and was not easily hardened by quenching. The people of the Middle East found that a much harder product could be created by the long term heating of a wrought iron object in a bed of charcoal, which was then quenched in water or oil. The resulting product, which had a surface of steel, was harder and less brittle than the bronze it began to replace.

In China the first irons used were also meteoric iron, with archeological evidence for items made of wrought iron appearing in the northwest, near Xinjiang, in the 8th century BC. These items were made of wrought iron, created by the same processes used in the Middle East and Europe, and were thought to be imported by non-Chinese people.

In the later years of the Zhou Dynasty (ca 550 BC), a new iron manufacturing capability began because of a highly developed kiln technology. Producing blast furnaces capable of temperatures exceeding 1300 K, the Chinese developed the manufacture of cast, or pig iron.

If iron ores are heated with carbon to 1420-1470 K, a molten liquid is formed, an alloy of about 96.5% iron and 3.5% carbon. This product is strong, can be cast into intricate shapes, but is too brittle to be worked, unless the product is decarburized to remove most of the carbon. The vast majority of Chinese iron manufacture, from the Zhou dynasty onward, was of cast iron. Iron, however, remained a pedestrian product, used by farmers for hundreds of years, and did not really affect the nobility of China until the Qin dynasty (ca 221 BC).

Cast iron development lagged in Europe, as the smelters could only achieve temperatures of about 1000 K. Through a good portion of the Middle Ages, in Western Europe, iron was still being made by the working of sponge iron into wrought iron. Some of the earliest casting of iron in Europe occurred in Sweden, in two sites, Lapphyttan and Vinarhyttan, between 1150 and 1350 AD. There are suggestions by scholars that the practice may have followed the Mongols across Russia to these sites, but there is no clear proof of this hypothesis. In any event, by the late fourteenth century, a market for cast iron goods began to form, as a demand developed for cast iron cannonballs.

Early iron smelting (as the process is called) used charcoal as both the heat source and the reducing agent. In 18th century England, wood supplies ran down and coke, a fossil fuel, was used as an alternative. This innovation by Abraham Darby supplied the energy for the Industrial Revolution.

Iron is extracted from the orehematite, which is mainly Fe2O3. The iron is separated from the ore by reduction with carbon - a more reactive element. This process takes place in a blast furnace at temperatures of about 2000°C.

The iron is tapped off as a liquid. However it is still not pure, the main impurity being sand (silicon dioxide). This is reacted with limestone (CaCO3) to form slag.

First the limestone (3) is decomposed to calcium monoxide and carbon dioxide:

CaCO3 → CaO + CO2

Then the calcium monoxide reacts with the silicon dioxide forming slag, which can be used for road-building or fertiliser:

CaO + SiO2 → CaSiO3

Approximately 1100Mt (million tons) of iron ore was produced in the world
in 2000, with a gross market value of approximately 25 billion US dollars. While ore production occurs in 48 countries, the five largest producers were China, Brazil, Australia, Russia and India, accounting for 70% of world iron ore production. The 1100Mt of iron ore was used to produce approximately 572Mt of pig iron.

Iron (VI) is also known, if rare. In the form of potassium-ferrate (K2FeO4) it is a selective oxidizer for primary alcohols. The solid is only stable under vacuum and dark purple in both (caustic) solution and as a solid.

Animals incorporate iron into the heme complex, an essential component of proteins involved in redox reactions, including respiration. Inorganic iron is also found in the iron-sulfur clusters of many enzymes, such as nitrogenase (Synthesis of ammonia from nitrogen and hydrogen) and hydogenase. A class of non-heme-iron enzymes is responsible for a wide range of functions within several life forms, such as methane monooxygenase (oxidizes methane to methanol), ribonucleotide reductase (reduces ribose to desoxyribose; DNA biosynthesis), hemerythrins (oxygen transport and fixation in marine invertebrae) and purple acid phosphatase (hydrolysis of phosphateesters).

Iron distribution is heavily regulated in mammals[1]. For example, when the body is fighting a bacterial infection, the body "hides" iron so it cannot be used by the bacteria (see transferrin).

Iron has four naturally-occurring stable isotopes, 54Fe, 56Fe, 57Fe and 58Fe.
The relative abundances of the Fe isotopes in nature are approximately 54Fe (5.8%), 56Fe (91.7%), 57Fe (2.2%) and 58Fe (0.3%). 60Fe is an extinct radionuclide which had a long half-life (1.5 Myr). Much of the past work on measuring the isotopic composition of Fe has centered on determining 60Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation.
The isotope 56Fe is of particular interest to nuclear scientists as it represents the most stable nucleus possible. It is not possible to perform fission or fusion on 56Fe and still liberate energy. This does not hold true for any other element.
In phases of the meteorites Semarkona und Chervony Kut a correlation between the concentration of 60Ni, the daughter product of 60Fe, and the abundance of the stable iron isotopes could be found which is evidence for the existence of 60Fe at time formation of solar system. Possibly the energy released by the decay of 60Fe contributed, together with the energy released by decay of the radionuclide 26Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of 60Ni present in extraterrestrial material may also provide further insight into the origin of the solar system and its early history.
Of the stable isotopes, only 57Fe has a nuclear spin (-1/2). For this reason, 57Fe has application as a spin isotope in chemistry and biochemistry.

The lethal dose of iron in a 2 year old is about 3 grams of iron. One gram can induce severe poisoning. There are reported cases of children being poisoned by consuming between 10-50 tablets of ferrous sulfate over a few hour period. Overconsumption of iron is the single highest cause of death in children by unintentional ingestion of pharmaceuticals.

If iron intake is uncontrollably high, a disease results, called hemochromatosis. The iron excess accumulates in the liver, with siderosis and organ damage as a result. For this reason, people shouldn't take iron supplements unless they suffer from iron deficiency and have consulted a doctor. Blood donors are at special risk of low iron levels and are often recommended to supplement their iron intake.